The migrations of Trichiurus japonicus in response to Kuroshio Current in northern Taiwan during winter based on satellite measured data

Abstract

The marginal sea off northern Taiwan is considered as the major fish farm due to its complex environment conditions mixed with Kuroshio Current, shelf water, upwelling, and alternating monsoon winds. More than 90% of the catch of cutlassfish (Trichiurus japonicus), a key commercial species off northern Taiwan, occurs from October to March, exhibiting pronounced seasonal and spatial variability since 2014. These variations in fish catch are likely influenced by regional oceanographic dynamics, yet their relationships have not been systematically investigated. This study analyzes monthly commercial landing data of fish catch and the related environmental parameters to identify the domain factors affecting the eastward migration off northern Taiwan starting in December during 2014 to 2019. Statistical analyses of satellite observations indicate that fish catch migration from December to March is strongly connected to the 22 degrees C sea surface temperature, which corresponds to the onset of colder continental shelf waters during early water. In contrast, the associations with other environmental parameters are relative weak. The primary fishing areas (>50% of monthly maximum) display wider variations in December 2016 and 2019 due to high temperature variability, whereas these areas remain highly consistent across years during the other winter months. Empirical orthogonal function analyses of sea level anomaly and geostrophic velocity reveal two characteristic modes of Kuroshio Current - an intrusion type and an offshore type - along with their associated shelf circulation northeast of Taiwan. In December, the intrusion type dominates in 2014, 2015, 2017, and 2018, whereas the offshore type prevails in 2016 and 2019; the latter coincide with years exhibiting wider spatial variations in fishing areas. Composites from the two periods indicate that shifts in the dominant Kuroshio mode during December alter the surface and upper ocean thermal and circulation structures west of 122 degrees E, thereby driving the year-to-year displacement of fishing areas.